In the magnetic storage of broad-band signals, in particular video signals, on magnetic tape, the signals are recorded and read out or reproduced by one or more rotating transducers disposed on the circumference of a head wheel or disk. In the recording mode of operation, the excitation of the transducers must be adjusted to an optimal value, such that in the reproducing mode, a high-frequency output signal at the highest possible level, with the least possible background noise, is produced. Depending on the design and wearing of the transducers, and depending on the type of tape used, the optimized values can vary, necessitating later readjustment.
In German Patent Disclosure Document No. DE-OS No. 32 32 610 to which U.S. Pat. No. 4,524,396, Schulz et al corresponds, of the same Assignee, an amplifier system for a magnetic tape recording and reproduction unit has already been described, in which reproduction transducers, rotating windings of an inductive transformer system and component assemblies of the pre-amlifiers associated with the reproduction transducers are provided on a rotating disk. Each pre-amplifier has two symmetrical amplifier branches, each having a series circuit comprising a voltage-coupled grounded emitter circuit and grounded collector circut, the outputs of which are connected to the rotating winding of the inductive transformer system. In a further development of this apparatus, the magnetic heads or transducers are provided with a winding having a center tap, which is connected to a fixed voltage. This amplifier system relates, however, to a reproduction preamplifier.
From German Patent Disclosure Document No. 20 23 176, a differential amplifier having two transducers of the same conductivity type is known, the emitters of which are connected to one another forming a quasi-complementary circuit and the bases of which, in a bridge circuit, are connected to the end terminals of a coil having a center tap. The bases of the quasi-complementary transistor pair are cross-connected to the collectors via capacitors. The properties of an unstable multivibrator are thereby supposed to be imparted to the differential amplifier. By decreasing the conductivity of one transistor, an increase in the voltage at the base of the other transistor is brought about, resulting in a reduction in the voltage at its collector. This transferred via the other capacitor to the base of the first transistor and its conductivity is decreased still more. This relates, however, to a differential amplifier without power adjustment via the supply voltage.
A differential stage as the first stage results in a high zero-point constancy. A circuit of that kind has the disadvantage that at high input voltages, each of the amplifier stages can be driven up to saturation. If the input voltage subsequently drops abruptly from a high value to a very low value (in analog signals), corresponding to a 1/0 transition in digital signals, the result is a dead period as a consequence of charge carrier storage effects in the saturated transistors.
In differential amplifiers having two bipolar transistors, the input impedance is relatively low. The difference between the current amplification factors can amount to from 10 to 20%. This necessitates provisions for reducing the imbalance.